David wrote:

When I got it, two oil bath resistors were *way* off value (202 and 100 ohms 
low respectively), and the S2 shunt resistors were about 1.3 ohms high which 
was far enough off to prevent S2 calibration.   There were were also sundry 
other problems like two open circuits in the final decade, a badly worn trim 
pot, and sundry wires broken at solder joints.


The resistors in a 720A -- even the pad resistors -- do not drift that much by themselves under normal use. Trim pots do not get used enough to wear out under normal operation. If you have some that are worn, the instrument has been abused. This is further indicated by the broken wires. I have *never* seen a broken wire in a 720A.

Unfortunately, it seems clear that your 720A was traumatized in its earlier life, and some of the divider resistors were damaged (in addition to other probable damage, based on your reports). There really isn't anything you can do but replace the bad resistors with equal or better parts. Trimming them as you have done is just a temporary band-aid. Resistors that have been traumatized will never allow the instrument to meet its time and temperature drift specifications, and you will be going back inside to replace (and re-replace) pad resistors every time you want to use it.

Worse, the problem goes deeper than that. Even replacing the resistors you know are bad will not restore the instrument to its accuracy and stability specs. The techniques you are using to identify the bad resistors will only identify ones that have been grossly damaged -- they will not identify all of the resistors that have been damaged and are preventing the instrument from meeting its specifications.

Further, the very act of replacing resistors in a 720A is likely to cause additional damage -- to the resistors you are soldering, the resistors attached to them, and the infrastructure of the instrument. Add to this the fact that only a few qualified replacement resistors will cost more than a properly-working used 720A, and you can see that repairing a traumatized 720A -- if you can do it at all -- is not cost-effective for an amateur.

The only 720As I've ever seen that worked correctly after being traumatized had been repaired by Fluke at astronomical cost. I hate to say it, but if you expect to use a 720A to calibrate meters of 6-1/2 digits and up, you will eventually learn that you need to replace the one you have.

Best regards,

Charles


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